The other related cleanup task is to revisit whether some of those interfaces (namely SyncService) can be split into smaller ones.

Initial survey:

SyncService and KeyedService are fine.

Identity-related things: See also  bug 825190 .
- OAuth2TokenService::Observer might get removed mid-term.
- GaiaCookieManagerService::Observer is only for detecting cookie jar stats (mismatch, empty), I think this should get obsolete with DICE.
- IdentityManager::Observer should be fine, could maybe be extracted into a new "SyncAccountTracker" or something like that.

SyncEngineHost: This is basically an observer for the SyncEngine, and it has lots of methods. Maybe the whole SyncEngine lifetime can be extracted into another class.

SyncPrefObserver: Only used for detecting changes to the "sync managed" pref, i.e. whether Sync is forced off by policy. Probably fine.

DataTypeManagerObserver: Gets notified of OnConfigureStart and OnConfigureDone. Doesn't seem like this can be easily extracted.

UnrecoverableErrorHandler: This is passed to the SyncEngine, maybe it can be merged into SyncEngineHost (and extracted along with that)?

https://crrev.com/c/1057508 has extraced OAuth2TokenService::Observer and IdentityManager::Observer into a separate class (SyncAuthManager).

I don't think there's any low-hanging fruit left here:
- The identity/OAuth things have been pulled out.
- SyncEngineHost and DataTypeManagerObserver won't be easy to pull out.
- UnrecoverableErrorHandler might be possible to remove, but it's only used for the Directory, so it'll hopefully become obsolete soonish anyway.
- GaiaCookieManagerService::Observer will become obsolete once DICe is fully rolled out.

So let's close this.